WL#1490: Optimizer hint for specifying key length to use

We now have Hints which allow you to specify which index to use for the table,
to work around wrong optimizer decisions (which are possible with any cost model)
For example in some cases optimizer prefers to use shorter "ref" instead of
"range" for the full index.
With index merge and loose index scan choices will be even more complex.
We need to be able to specify the keyparts which optimizer shall use.
For example:
FORCE INDEX(a(col1,col2)) to use both columns, if possible
FORCE INDEX(a(col1)) to use only first column - helps if the "range" on
second index part is not selective.
Different syntax is possibe of course :)